NAME¶

runqlat - Trace long process scheduling delays.

SYNOPSIS¶

runqslower [-p PID] [min_us]

DESCRIPTION¶

This measures the time a task spends waiting on a run queue (or equivalent scheduler data structure) for a turn on-CPU, and shows occurrences of time exceeding passed threshold. This time should be small, but a task may need to wait its turn due to CPU load. The higher the CPU load, the longer a task will generally need to wait its turn.

This tool measures two types of run queue latency:

1. The time from a task being enqueued on a run queue to its context switch and execution. This traces ttwu_do_wakeup(), wake_up_new_task() -> finish_task_switch() with either raw tracepoints (if supported) or kprobes and instruments the run queue latency after a voluntary context switch.

2. The time from when a task was involuntary context switched and still in the runnable state, to when it next executed. This is instrumented from finish_task_switch() alone.

The overhead of this tool may become significant for some workloads: see the OVERHEAD section.

This works by tracing various kernel scheduler functions using dynamic tracing, and will need updating to match any changes to these functions.

Since this uses BPF, only the root user can use this tool.

REQUIREMENTS¶

CONFIG_BPF and bcc.

OPTIONS¶

-h

Print usage message.

-p PID

Only show this PID (filtered in kernel for efficiency).

min_us

Minimum scheduling delay in microseconds to output.

EXAMPLES¶

Show scheduling delays longer than 10ms:

# runqslower

Show scheduling delays longer than 1ms for process with PID 123:

# runqslower -p 123 1000

FIELDS¶

TIME

Time of when scheduling event occurred.

COMM

Process name.

PID

Process ID.

LAT(us)

Scheduling latency from time when task was ready to run to the time it was assigned to a CPU to run.

OVERHEAD¶

This traces scheduler functions, which can become very frequent. While eBPF has very low overhead, and this tool uses in-kernel maps for efficiency, the frequency of scheduler events for some workloads may be high enough that the overhead of this tool becomes significant. Measure in a lab environment to quantify the overhead before use.

SOURCE¶

This is from bcc.

https://github.com/iovisor/bcc

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool.

OS¶

Linux

STABILITY¶

Unstable - in development.

AUTHOR¶

Ivan Babrou

SEE ALSO¶

runqlen(8), runqlat(8), pidstat(1)